Invisible Axion Shines a Light on Neutrino Mass Generation

[ Jun 6, 2018 ]

Abstract:
The axion is an elementary particle that was predicted to exist as a consequence of the Peccei–Quinn solution to the strong charge+parity problem. To relate Peccei-Quinn symmetry with the neutrino mass problem, a Kanazawa University researcher extended the Kim–Shifman–Vainshtein–Zakharov invisible axion model to include a dark matter candidate and generate small neutrino masses. This occurred through a one-loop effect as a result of breaking Peccei–Quinn symmetry.

Kanazawa, Japan – The cosmos is full of matter that forms impressive astronomical bodies that delight stargazers, but there's much more to the universe than stars, planets, and galaxies.

A very small proportion of matter is visible, detectable, or even understandable. Physicists around the world will therefore continue to build and develop theoretical models that help to explain the composition of the universe.

In Japan, Daijiro Suematsu at Kanazawa University's Institute for Theoretical Physics extended the Kim–Shifman–Vainshtein–Zakharov (KSVZ) invisible axion model to include a dark matter candidate and generate small neutrino masses. The extended model may be considered as a well-motivated simple framework in high-energy regions for the original scotogenic (i.e., created by dark matter) model. He reported his work in The European Physical Journal C.

KSVZ is one of two forms of the invisible axion. The axion is a hypothetical fundamental particle, which means that it's not composed of other particles. It was predicted to exist as a consequence of the Peccei–Quinn solution to the strong charge+parity (CP) problem, which is one of the perplexing unsolved problems in physics. Peccei–Quinn symmetry extended to the neutrino sector forbids tree-level neutrino mass generation and its subgroup, Z2 symmetry, guarantees dark matter stability.

Given that neutrinos are massless at the tree level, to generate mass, Suematsu considered them at the one-loop level, which required an innovative, albeit simple, approach. By extending the KSVZ invisible axion model, Suematsu broke Peccei–Quinn symmetry, and this allowed neutrino mass to be generated. "The resulting low-energy effective model is free from the strong CP problem," Suematsu says.

For this research, an inert doublet scalar and three right-handed neutrinos were added to the KSVZ model. After breaking Peccei–Quinn symmetry, the model was reduced to the original scotogenic neutrino mass model with the remaining effective Z2 symmetry. This type of symmetry guarantees the stability of the lightest neutral component of the inert doublet scalar to provide a dark matter candidate. The neutrino mass was generated through a one-loop effect as a result of the breaking of Peccei–Quinn symmetry through a nonrenormalizable interaction.

"Because this simple extension can relate the strong CP problem to the origin of neutrino masses and dark matter," Suematsu explains, "it may be a promising extension of both the KSVZ model and the scotogenic model."